Hot melt adhesives and moisture cure urethane adhesives are well known generic adhesive classes. Hot melt adhesives generally comprise thermoplastic materials which can be conveniently applied by extruding the adhesive composition at elevated temperatures onto a workpiece to which it is desired to fix another workpiece. The adhesive bond increases in integrity as the composition cools. Hot melt adhesives generally have good green strength, i.e., initial strength before complete setting of the bond, and they have the additional benefits of being easy to handle and free of solvents which must be evacuated during the setting or curing process of the adhesive. However, hot melt adhesives have the drawback that after cooling to form a bond line, the thermoplastic compositions can be temperature sensitive. In other words, the bonding mass can lose bond strength as the temperature of the workpiece and the bond line increase. Further, hot melt adhesives tend to be physically unstable in the presence of hydrocarbon solvents and some organic compositions.
In contrast, many curable urethane adhesives have little green strength. Thus, after the application of a curable urethane adhesive, the structure to be bonded must be externally supported until the urethane can cure to a strong, resilient, highly crosslinked bond line. Additionally, solvents may be used in these adhesives which may need to be evaporated during cure. However, once cured, these urethane adhesives have high tensile strength and have little or no temperature sensitivity. They also offer good resistance to many solvents after cure.
Clearly, a single adhesive composition displaying both curing and hot melt properties is a desirable goal, since the resulting adhesive, in theory, could possess quick, high strength green bonding and strong, crosslinked cured adhesive bonding.
U.S. Pat. No. 4,623,709 discloses polyurethane prepolymers in which polyhydric alcohols are initially reacted with the faster reacting isocyanate group of an asymmetrical diisocyanate with the slowly reacting group remaining intact, after which the reaction products are combined with a symmetrical diisocyanate of which the equally reactive isocyanate groups again react more quickly than the slowly reacting groups of the first polyfunctional isocyanate compound. The products obtained by this process are allegedly distinguished by a low residual monomer content.
U.S. Pat. No. 5,441,808 discloses hot melt adhesive compositions combining the characteristics of thermoplastic hot melt adhesives and reactive adhesives and are prepared by blending a polyester polyether thermoplastic elastomer with a polyisocyanate prepolymer.
U.S. Pat. No. 3,931,077 discloses reactive, high viscosity hot melt adhesive compositions comprising a specific high viscosity reactive urethane prepolymer, a specific ethylene-vinyl acetate thermoplastic polymer and a phenolic or abietic acid-type tackifying resin.
U.S. Pat. No. 4,585,819 discloses the combination of an isocyanate prepolymer, a thermoplastic polyurethane or polyester and a synthetic resin which may be a ketone resin, a hydrogenation product of acetophenone condensation resins, and mixtures thereof.
U.S. Pat. No. 4,775,719 discloses a thermally stable, hot melt moisture cure polyurethane adhesive composition comprising an ethylene-vinyl monomer having an ethylene content of 55 wt % or less, a polyisocyanate prepolymer, and an aromatic tackifying resin.
U.S. Pat. No. 4,808,255 discloses a thermally stable, moisture cure urethane hot melt adhesive comprising a thermoplastic polymer, a compatible, curing urethane polyalkylene polyol prepolymer, and a tackifying agent.
U.S. Pat. No. 4,820,368 discloses a thermally stable, moisture cure urethane hot melt adhesive comprising a thermoplastic polymer, a compatible, curing urethane polyester polyol prepolymer, and a tackifying agent.